[ineedasername]

Not quite. Let's say I know you're cheating on your partner: I can tell the partner, and that's legally fine. But if I say "I'm going to tell the partner if you don't pay me $7500" then that is not fine, even though the first action is legitimate. Coercion really is quite a bit about the second part as well.

I'm not sure if this rule would cover all coercion/blackmail, but a rule like the following is probably a good guideline: If the first part negatively impacts the "victim" while the second part positively impacts the other person, it's might be getting close to coercion territory.

Let's take your cake example: The person with the cake isn't really negatively impacted. If they don't like the cake, they aren't materially harmed by someone else eating it. Although even there, context matters: Let's say you're a baker, and you sell cakes, even ones that you don't like yourself (maybe you hate buttercream icing). Taking your cake and eating it when you might otherwise have sold the cake and made money would be a problem.